Patients develop acute lung injury upon exposure to an injurious stimulus. Acute lung injury is a clinical syndrome characterized by airspace flooding with the development of an intrapulmonary shunt resulting in hypoxemic respiratory failure. The alveolar epithelium is the primary barrier preventing the formation of edema and intrapulmonary shunt. Recent studies have indicated that apoptosis may be an important mechanism underlying the pathogenesis of acute lung injury. A commonly used laboratory model of lung injury is hyperoxia (95%-100% O2). Hyperoxia causes alveolar epithelial cell death, edema, inflammation and death of the animal within 3 days. The apoptotic pathway is executed by caspases, a family of cysteine proteases. Alveolar epithelial cells can undergo activate caspases through either a mitochondrial or a receptor dependent pathway. However, it is not known whether either apoptotic pathway is important in the development of acute lung injury. Furthermore, it is unclear whether caspase inhibition following the initiation of apoptosis through either a death receptor or mitochondrial dependent pathway can prevent cell death and preserve the function of alveolar epithelial cells. We hypothesize that the loss of alveolar epithelial cells due to apoptosis is an initiating event in the pathology associated with hyperoxia. The focus of this application is to determine whether mitochondrial or receptor dependent pathways regulate hyperoxia induced lung injury. In addition we will examine the efficacy of genetic strategies to inhibit caspase activation in preventing lung injury and preserving epithelial function during hyperoxia. Collectively these studies will provide important insight into the mechanisms underlying acute lung injury and may lead to the development of novel therapeutic strategies both for patients with established ARDS and those at risk for developing ARDS.